This invention relates to an improved catalyst system and to processes for using the catalyst system. In particular, to a catalyst system utilized in high temperature reactions having flow-through radiation shielding as well as to a process for preparing hydrogen cyanide using the same.
The maintenance of system energy in a high temperature catalytic reaction is important. For instance, in the manufacture of hydrogen cyanide by the ammoxidation of methane a high reaction temperature is required to maintain the highly endothermic cyanide formation reaction. In the Andrussow method for preparing hydrogen cyanide (see U.S. Pat. No. 1,934,838), ammonia, oxygen-containing gas such as air, and hydrocarbon gases such as methane are fed to a reaction system, at ambient or elevated temperature. The reactants are then reacted in the presence of a Pt-containing catalyst at temperatures of 1000xc2x0 C. to 1400xc2x0 C. to produce hydrogen cyanide.
In such high temperature catalytic reactions, a considerable amount of system energy may be lost as radiant energy. One mechanism for loss of radiant energy, in high temperature catalytic reactions, occurs when a metal containing catalyst material is utilized in the reaction. Such a catalyst material will glow as a result of the high temperatures of the reaction. Consequently, energy in the form of radiant energy is emitted from the glowing catalyst. Such radiant energy can escape from the reaction zone wherein it is lost to unproductive heating of upstream equipment, refractory, cooling jackets and the surrounding environment.
In the aforementioned Andrussow process for preparing hydrogen cyanide, the system energy demand is primarily met through combustion of a portion of the hydrocarbon/ammonia reactant feed gases. Accordingly, the net result of such loss of radiant energy is an increase in consumption of hydrocarbon/ammonia for combustion to maintain the system energy. Consequently, either additional hydrocarbon/ammonia feed gas is utilized or the yield of hydrogen cyanide product decreases because less reactants are available to the reaction because of combustion. As a result, there is an increase in manufacturing costs because an increased proportion of reactants are used to fulfill the reaction system""s energy demands. Accordingly, there is a constant demand for means to decrease reactant combustion thereby improving hydrogen cyanide yield.
Pre-heating of reactant gases has been disclosed as a means of decreasing reactant combustion and improving hydrogen cyanide yield. In U.S. Pat. No. 3,104,945 a process for preparing hydrogen cyanide is disclosed where air, methane and ammonia are preheated and mixed before being reacted in the presence of a platinum group metal catalyst. A decrease in oxygen and methane usage and increased hydrogen cyanide yield on methane and ammonia is disclosed.
Prevention of heat loss in hydrogen cyanide production is disclosed in U.S. Pat. No. 3,215,495 using a combination of two refractory fiber blankets having a layer of refractory particles disposed between the two blankets. The fiber blanket combination is disposed directly on the catalyst and provides a reduction in heat loss from the system resulting in decreased reactant combustion.
The present inventors have now discovered a novel catalyst system which employs flow through radiation shielding and a process for preparing hydrogen cyanide using the same wherein the following advantages are provided:
(1) radiant-energy losses from the reaction zone are minimized thereby lowering the reactant feed proportion which must be combusted to maintain the endothermic hydrogen cyanide formation reaction;
(2) higher hydrocarbon/ammonia yields of hydrogen cyanide are realized as a direct result of the shift away from combustion of reactants;
(3) the total volumetric load per unit of product hydrogen cyanide is reduced thus increasing production capacity more economically than other methods such as oxygen enrichment;
(4) improved flow distribution into the reaction zone, providing more-uniform catalyst temperatures which lead to higher yields;
(5) mechanical protection of the catalyst from yield-reducing process contaminants, as with filtration;
(6) lower upstream equipment surface-temperatures, which retards pre-combustion of feed gases, helps to minimize the potential for reverse flame-front propagation and associated deflagrations, reduces equipment cooling requirements, and allows for simplified mechanical designs vs. high-temperature requirements;
(7) reduced reaction system heat capacitance, allowing for faster heating at start-up and quicker cooling at shutdown of the reaction system (improved cycle time), faster cool-down also retards formation of volatile PtO2, which is one mechanism for catalyst loss;
(8) longer life of downstream waste heat recovery exchangers as a result of lower total heat load for the same HCN production rate;
(9) reduced mass flow of CO2/CO combustion products in the exit gas results in reduced bicarbonate or carbonate formation in caustic absorbers used in formation of sodium cyanide; and
(10) less CO2 mass flow, when absorption based ammonia recovery systems are used, such as in U.S. Pat. Nos. 2,590,146; 3,104,945; 4,094,958; and 4,128,622; reduces formation of ammonium carbamate which can interfere with the stable operation of ammonia purification/distillation columns and is also highly corrosive to carbon steel thereby by forming ferrous/ferric compounds in the recycle ammonia stream which are damaging ammonia compression equipment and may also poison the catalyst.
In one aspect of the present invention, there is provided a process for preparing hydrogen cyanide, including the steps of: (A) feeding reactants including at least one hydrocarbon, at least one nitrogen containing gas and at least one oxygen containing gas into a reactor; (B) pre-heating the reactants by passing the reactants through an at least partially heated radiation shield, comprising one or more pieces of a flowthrough ceramic material, into a reaction zone, the radiation shield having been at least partially heated by absorbing at least a portion of radiant energy produced in the reaction zone; (C) reacting the reactants at a temperature of 800xc2x0 C. to 1400xc2x0 C. in the presence of a platinum group metal catalyst disposed within the reaction zone to produce hydrogen cyanide; (D) monitoring the temperature of the reaction zone; and (E) adjusting the oxygen content of the reactants to maintain the reaction temperature within a range of 800xc2x0 C. to 1400xc2x0 C.
In a second aspect of the present invention, there is provided a catalyst system for use in a high temperature chemical process, including: (A) a reaction zone having a catalyst disposed therein; (B) a radiation shield including one or more pieces of a flowthrough ceramic material disposed upstream of the reaction zone for (i) absorbing at least a portion of radiant energy produced in the reaction zone, and (ii) transferring heat formed from absorbing the radiant energy to reactants flowing therethrough into the reaction zone; and (C) one or more temperature sensing devices disposed within the reaction zone.